Vibration damping rubber composition

ABSTRACT

A vibration damping rubber composition having an excellent vibration damping performance includes the following (A) to (C) as essential components:
     (A) a diene-based rubber;   (B) a vulcanizing agent of a bismaleimide compound represented by the following formula (1):   

     
       
         
         
             
             
         
       
         
          [wherein X denotes a hydrocarbon group having an aromatic ring in its molecular structure or an aromatic hydrocarbon group and may or may not have a substituent; and R 1  to R 4  may be the same or different and each denote a hydrogen atom, an alkyl group, —NH 2 , or —NO 2 ], wherein the vulcanizing agent is sulfur-free; and 
         (C) a vulcanization accelerator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to vibration damping rubber compositions,more specifically, relates to vibration damping rubber compositions usedon engine mounts or the like, which function as a support for automobileengines and suppress vibration transmission.

2. Description of the Art

In conventional vibration damping rubber compositions, for example,rubber compositions prepared by adding a sulfur-vulcanizing agent and avulcanization accelerator to a rubber material, such as a natural rubberand a synthetic rubber, have been used. However, such rubbercompositions have a disadvantage that the rubbers harden due tolong-term service (heat aging) causing a decrease in the vibrationdamping performance.

The causes of such heat aging deterioration are thought as follows: (a)an increase in cross-linking density by a change in a form of sulfurcross-linking from a polysulfide bond to a disulfide or monosulfidebond; (b) oxidative deterioration of the rubber material itself; and (c)cross-linking caused by the sulfur-vulcanizing agent.

These problems have been dealt with by, for example, (a) forming amonosulfide-rich cross-link, (b) adding an antioxidant, and (c)minimizing the blend ratio of the sulfur-vulcanizing agent. However, thehardening of the rubber due to long-term service (heat aging) could notbe sufficiently suppressed. Therefore, the vibration damping performancewas poor.

Consequently, in order to solve the problems in the vibration dampingperformance, a variety of rubber compositions using a bismaleimide-basedcompound as the vulcanizing agent have been proposed (for example,

Japanese Unexamined Patent Publication No. 2-284935,

Japanese Unexamined Patent Publication No. 4-136049, and

Japanese Patent Publication No. 7-122006).

However, each of the rubber compositions described in theabove-mentioned three Patent Publications includes a sulfur-vulcanizingagent along with a bismaleimide-based compound. Therefore, thecross-linking progresses by the sulfur-vulcanizing agent to harden therubber. Thus, such rubber compositions have a disadvantage thatsufficient vibration damping performance cannot be achieved.

The present invention has been made in view of the above-mentionedcircumstances, and it is an object of the present invention to provide avibration damping rubber composition which can achieve excellentvibration damping performance.

SUMMARY OF THE INVENTION

In order to achieve the above-mentioned object, a vibration dampingrubber composition according to the present invention includes thefollowing (A) to (C) as essential components:

-   (A) a diene-based rubber;-   (B) a vulcanizing agent of a bismaleimide compound represented by    the following formula (1):

-    [wherein X denotes a hydrocarbon group having an aromatic ring in    its molecular structure or an aromatic hydrocarbon group and may or    may not have a substituent; and R¹ to R⁴ may be the same or    different and each denote a hydrogen atom, an alkyl group, —NH₂, or    —NO₂], wherein the vulcanizing agent is sulfur-free; and-   (C) a vulcanization accelerator.

Namely, the present inventors have intensively studied in order toobtain a vibration damping rubber composition which can achieveexcellent vibration damping performance and have focused on avulcanization system wherein only a particular bismaleimide compound isused as a vulcanizing agent that is sulfur-free. The inventors havefound that in this vulcanization system a vulcanization accelerator isrequired to be used along with a bismaleimide compound for improvingcross-linking reactivity of the bismaleimide compound as the vulcanizingagent. As a result, the inventors have finally found that the desiredpurpose can be achieved by a vibration damping rubber compositionincluding a diene-based rubber, a particular bismaleimide compound as avulcanizing agent not containing elemental sulfur, and a vulcanizationaccelerator. Thus, the present invention has been completed.

The vibration damping rubber composition according to the presentinvention includes a particular bismaleimide compound not containingelemental sulfur as a vulcanizing. agent and includes a vulcanizationaccelerator. Therefore, the hardening of the rubber due to long-termservice (heat aging) can be sufficiently suppressed and excellentvibration damping performance can be achieved. Additionally, since thevibration damping rubber composition according to the present inventionincludes a particular bismaleimide compound as a vulcanizing agent, thecross-linking with the diene-based rubber is formed by a C—C bond ofwhich binding energy is larger than that of a sulfide bond in sulfurcross-linkage. Therefore, excellent heat resistance can be achieved.

With respect to the vulcanization accelerator, in particular, by using athiazole-based vulcanization accelerator, the cross-linking reactivitycan be improved.

Furthermore, with respect to the thiazole-based vulcanizationaccelerator, in particular, the cross-linking reactivity can be furtherimproved by using dibenzothiazyl disulfide (MBTS) or2-(4-morpholinodithio)benzothiazole (MDB).

Additionally, with respect to the vulcanization accelerator, inparticular, by using a thiuram-based vulcanization accelerator, a scorchtime can be increased. Consequently, excellent storage stability in anunvulcanized rubber condition can be achieved and rubber-burning duringthe vulcanization process can be further suppressed. As a result,excellent workability can be obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments according to the present invention will now be describedin detail.

The vibration damping rubber composition according to the presentinvention can be prepared by using a diene-based rubber (component A), aparticular bismaleimide compound (component B), and a vulcanizationaccelerator (component C).

In the present invention, it is the distinguishing characteristic thatthe vibration damping rubber composition includes the particularbismaleimide compound (component B) alone as a vulcanizing agent notcontaining elemental sulfur. Here, the elemental sulfur means sulfurwhich is commonly used in rubber vulcanization.

Examples of the diene-based rubber (component A) include, but are notlimited to, natural rubbers (NR), isoprene rubbers (IR), butadienerubbers (BR), styrene-butadiene rubbers (SBR), andacrylonitrile-butadiene rubbers (NBR). These rubbers may be used aloneor in combination thereof. Among these rubbers, NR is preferable fromthe viewpoints of vibration damping performance and durability.

The particular bismaleimide compound (component B) used with thediene-based rubber (component A) is represented by the following formula(1):

[wherein X denotes a hydrocarbon group having an aromatic ring in itsmolecular structure or an aromatic hydrocarbon group and may or may nothave a substituent; and R¹ to R⁴ may be the same or different and eachdenote a hydrogen atom, an alkyl group, —NH₂, or —NO₂].

In formula (1), X denotes a hydrocarbon group having an aromatic ring inits molecular structure or an aromatic hydrocarbon group. Examples ofthe hydrocarbon group include alkylene groups such as a methylene group.The number of the aromatic ring of the hydrocarbon group is not limitedto one and may be two or more and each aromatic ring may be bonded by—O—, —S—, —SS—, SO₂— or the like.

In formula (1), X may have a substituent. For example, the hydrocarbongroup, the aromatic group or the aromatic hydrocarbon group may have asubstituent. Examples of the substituent include alkyl groups(preferably, lower alkyl groups such as a methyl group, an ethyl group,and a propyl group), —NO₂, —NH₂, —F, —Cl and —Br.

In formula (1), examples of the alkyl group represented by R¹ to R⁴include lower alkyl groups having 1 to 5 carbon atoms such as a methylgroup, an ethyl group, and a propyl group.

Examples of the particular bismaleimide compound (component B) includeN,N′-m-phenylenedimaleimide, N,N′-(4,4,-diphenyl-methane)bismaleimide,bis(3-ethyl-5-methyl-4-maleimidephenyl)methane, 2,2′-bis[4-(4-maleimidephenoxy) phenyl]propane, m-phenylenebis(methylene)bismaleimide, andm-phenylenebis(methylene)biscitraconimide represented by the followingchemical formulae (2) to (7), respectively.

Among the above-mentioned particular bismaleimide compounds (componentB), N,N′-m-phenylenedimaleimide represented by chemical formula (2) canbe prepared, for example, as follows; Adding a predetermined amount ofm-phenylenediamine dropwise to a chloroform solution of anhydrous maleicacid; mixing the resulting bis(maleic acid) m-phenyleneamide,triethylamine, and sodium acetate with acetone to yield the targetcompound.

The blend ratio of the particular bismaleimide compound (component B) ispreferably 0.3 to 7 parts by weight (hereinafter referred to as simply“parts”) and more preferably 1 to 5 parts per 100 parts of thediene-based rubber (component A). Namely, when the blend ratio of thecomponent B is lower than 0.3 parts, the cross-linking reactivity tendsto decrease. Reversely, when the blend ratio of the component B ishigher than 7 parts, the physical properties (tensile strength at breakand elongation at break) decrease; which may cause a decrease indurability.

Any vulcanization accelerator (component C) can be used with thecomponent A and the component B. Examples of the vulcanizationaccelerator include thiazole compounds, sulfenamide compounds, thiuramcompounds, aldehyde ammonia compounds, aldehyde amine compounds,guanidine compounds, and thiourea compounds. These vulcanizationaccelerators may be used alone or in combination thereof. Among them,the thiazole-based vulcanization accelerator is preferable because ofits excellent cross-linking reactivity. Additionally, a combination ofthe thiazole-based vulcanization accelerator and the thiuram-basedvulcanization accelerator is preferable because of its excellentphysical properties (tensile strength at break and elongation at break).

Examples of the thiazole-based vulcanization accelerator includedibenzothiazyl disulfide (MBTS), 2-(4-morpholinodithio)benzothiazole(MDB), 2-mercaptobenzothiazole (MBT), 2-mercaptobenzothiazole sodiumsalt (NaMBT), and 2-mercaptobenzothiazole zinc salt (ZnMBT). Thesethiazole-based vulcanization accelerators may be used alone or incombination thereof. Among them, MBTS and MDB are preferable because oftheir excellent cross-linking reactivity. In particular, MDB ispreferable because of its excellent physical properties (tensilestrength at break and elongation at break).

Examples of the sulfenamide-based vulcanization accelerator includeN-cyclohexyl-2-benzothiazyl sulfenamide (CBS).

Examples of the thiuram-based vulcanization accelerator includetetramethylthiuram disulfide (TMTD).

The blend ratio of the vulcanization accelerator (component C) ispreferably 0.3 to 7 parts and more preferably 0.5 to 5 parts per 100parts of the diene-based rubber (component A). Namely, when the blendratio of the component C is lower than 0.3 parts, the cross-linkingreactivity tends to decrease. Reversely, when the amount of thecomponent C is higher than 7 parts, the physical properties (tensilestrength at break and elongation at break) may decrease.

In addition to the above-mentioned components A to C, the vibrationdamping rubber composition according to the present invention mayproperly contain a vulcanizing aid, an antioxidant, a processing aid, asoftener, and the like, if necessary.

Examples of the vulcanizing aid include, but are not limited to, zincoxide (ZnO) and magnesium oxide. These vulcanizing aids may be usedalone or in combination thereof.

The blend ratio of the vulcanizing aid is preferably 1 to 15 parts andmore preferably 2 to 10 parts per 100 parts of the diene-based rubber(component A).

Examples of the antioxidant include carbamate antioxidants,phenylenediamine antioxidants, phenol antioxidants, diphenylamineantioxidants, quinoline antioxidants, imidazole antioxidants, and waxes.

The blend ratio of the antioxidant is preferably 1 to 7 parts and morepreferably 2 to 5 parts per 100 parts of the diene-based rubber(component A).

Examples of the processing aid include stearic acid, fatty esters, fattyacid amides, and hydrocarbon resins.

The blend ratio of the processing aid is preferably 1 to 5 parts andmore preferably 1 to 3 parts per 100 parts of the diene-based rubber(component A).

The vibration damping rubber composition according to the presentinvention can be prepared, for example, by blending a diene-based rubber(component A), a particular bismaleimide compound (component B), avulcanization accelerator (component C), and additives such as avulcanizing aid therein if necessary, and kneading the mixture by usinga mixing roll, a Banbury mixer, or the like.

The thus prepared vibration damping rubber composition according to thepresent invention can be suitably used as a vibration damping materialfor, but is not limited to, engine-mounts, stabilizer bushings, andsuspension bushings used in vehicles such as automobiles.

Vibration damping rubber using the vibration damping rubber compositionaccording to the present invention can be produced, for example, bypress-vulcanizing the thus prepared vibration damping rubber compositionunder predetermined conditions and forming the thus obtained productinto a predetermined shape.

EXAMPLES

Examples and Comparative Examples will now be described.

The materials shown below were prepared for the Examples and theComparative Examples.

Diene-Based Rubber

Natural rubber

Diene-Based Rubber

Butadiene rubber (BR)

Diene-Based Rubber

Styrene-butadiene rubber (SBR)

Antioxidant A

N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (OZONONE 6C: SeikoChemical Co., Ltd.)

Antioxidant B

2,2,4-trimethyl-1,2-dihydroquinoline (NONFLEX RD: Seiko Chemical Co.,Ltd.)

Carbon Black

FEF (Fast Extruding Furnace) type carbon black (SEAST SO: Tokai CarbonCo., Ltd.)

Softener

Naphthenic mineral oil

Bismaleimide Compound A

N,N′-m-phenylenedimaleimide (VULNOC PM: Ouchi-Shinko Chemical IndustrialCo., Ltd.) represented by chemical formula (2)

Bismaleimide Compound B

N,N′-(4,4,-diphenyl-methane)bismaleimide (BMI-S: Mitsui Fine Chemicals,Inc.) represented by chemical formula (3)

Bismaleimide Compound C

Bis(3-ethyl-5-methyl-4-maleimidephenyl)methane (BMI-70: K•I ChemicalIndustry Co., Ltd.) represented by chemical formula (4)

Bismaleimide Compound D

2,2′-bis[4-(4-maleimide phenoxy)phenyl] propane (BMI-80: K•I ChemicalIndustry Co., Ltd.) represented by chemical formula (5)

Bismaleimide Compound E

M-phenylenebis(methylene)bismaleimide represented by chemical formula(6)

Bismaleimide Compound F

M-phenylenebis(methylene)biscitraconimide represented by chemicalformula (7)

Vulcanizing Agent

Sulfur

Vulcanization Accelerator (MBTS)

Sanceler DM: Sanshin Chemical Industry Co., Ltd.

Vulcanization Accelerator (CBS)

Sanceler CM: Sanshin Chemical Industry Co., Ltd.

Vulcanization Accelerator (TMTD)

Sanceler TT: Sanshin Chemical Industry Co., Ltd.

Vulcanization Accelerator (MDB)

NOCCELER MDB: Ouchi Shinko Chemical Industrial Co., Ltd.

The vibration damping rubber compositions were prepared by using thesematerials as described below.

Examples 1 to 17 and Comparative Examples 1 and 2

Each material shown in Tables 1 to 3 below was blended at the ratiosshown in the Tables, and the resulting blend was kneaded using a Banburymixer to prepare vibration damping rubber compositions.

Characteristics of the thus prepared vibration damping rubbercompositions for the Examples and the Comparative Examples wereevaluated according to the following criteria. The Tables 1 to 3 alsoshow the results.

Workability

Workability of each vibration damping rubber composition was evaluatedby measuring scorch time (t₅) at a testing temperature of 125° C.according to the test method of JIS K 6300. The material was graded as ⊚when the scorch time (t₅) was longer than 13 minutes and was graded as ◯when the scorch time (t₅) was 13 minutes or less.

Initial Physical Property

Each vibration damping rubber composition was press-vulcanized at 160°C. for 30 minutes and then was punched out using a JIS No. 5 dumbbell toprepare vibration damping rubber sheets having a thickness of 2 mm. Thetensile strength at break, the elongation at break, and the hardness(JIS A) of each vibration damping rubber sheet were evaluated inconformity with JIS K 6251.

Heat Aging Property

Using vibration damping rubber sheets prepared by the same manner asdescribed above, the elongation at break. and hardness (JIS A) weremeasured after aging at 100° C. for 500 hours. The change rate (%) ofthe elongation at break was determined for the elongation at break afterheat aging while the difference from the initial hardness was determinedfor the hardness after heat aging.

Spring Change

Stress at 100% elongation of vibration damping rubber sheets prepared bythe same manner as described above was measured in conformity with JIS K6251 and 100% modulus change rate (%) was determined.

Durability

Each vibration damping rubber composition was press-vulcanized at 160°C. for 30 minutes to prepare test pieces and was subjected to a flexingtest in conformity with JIS K 6260 to measure the number of flexingcycles until the cracking size reached 5 mm.

TABLE 1 (parts by weight) Example 1 2 3 4 5 6 7 Natural rubber 100 100100 100  100  100  100  Stearic acid 1 1 1 1 1 1 1 ZnO 5 5 5 5 5 5 5Antioxidant A 2 2 2 2 2 2 2 Antioxidant B 2 2 2 2 2 2 2 Carbon black 4040 40 40  40  40  40  Softener 5 5 5 5 5 5 5 Bismaleimide Blend ratio 22 2 2 2 2  0.3 compound Type A B C D E F A Vulcanizing agent (sulfur) —— — — — — — Vulcanization accelerator (MBTS) 2 2 2 2 2 2 2 Vulcanizationaccelerator (CBS) — — — — — — — Vulcanization accelerator (TMTD) — — — —— — — Vulcanization accelerator (MDB) — — — — — — — Workability ◯ ◯ ◯ ◯◯ ◯ ◯ Initial Tensile strength at break 17 17 16 16  16  16  17 physical (MPa) property Elongation at break (%) 490 520 530 550  510 490  550  Hardness (JIS A) 55 54 53 53  54  55  53  Heat Change rate ofelongation −18 −16 −15 −19    −18    −19    −16    aging at break (%)Property Hardness change (from +1 +1 +1 +1   +1   +1   +1   the initialhardness) Spring change: 100% +22 +20 +19 +21    +21    +18    +18   modulus change rate (%) Durability Number of flexing cycles 150 180 180200≦   200≦   200≦   200≦   until the cracking size reaches 5 mm(×10000)

TABLE 2 (parts by weight) Example 8 9 10 11 12 13 14 Natural rubber 100 100 100 100  100  100 100  Stearic acid 1 1 1 1 1 1 1 ZnO 5 5 5 5 5 5 5Antioxidant A 2 2 2 2 2 2 2 Antioxidant B 2 2 2 2 2 2 2 Carbon black 40 40 40 40  40  40 40  Softener 5 5 5 5 5 5 5 Bismaleimide Blend ratio 7 22 2 2 2 2 compound Type A A A A A A A Vulcanizing agent (Sulfur) — — — —— — — Vulcanization accelerator (MBTS) 2 0.3 7 — — — — Vulcanizationaccelerator (CBS) — — — — — — — Vulcanization accelerator (TMTD) — — — 0.1 1 3 — Vulcanization accelerator (MDB) — — — — — — 2 Workability ◯ ◯◯ ⊚ ⊚ ⊚ ⊚ Initial Tensile strength at break 15  16 15 18  18  17 19 physical (MPa) property Elongation at break (%) 410  540 390 570  580 530 590  Hardness (JIS A) 56  53 56 53  55  57 57  Heat Change rate ofelongation −15    −18 −18 −20    −22    −27 −27    aging at break (%)property Hardness change (from +1   +1 +1 +1   +1   +2 +2   the initialhardness) Spring change: 100% +17    +18 +18 +20    +24    +28 +25   modulus change rate (%) Durability Number of flexing cycles 200≦   150180 200≦   200≦   180 200≦   until the cracking size reaches 5 mm(×10000)

TABLE 3 (parts by weight) Comparative Example Example 15 16 17 1 2Natural rubber 50 50  100  100 100 Butadiene rubber 50 — — — — SBR — 50 — — — Stearic acid 1 1 1 1 1 ZnO 5 5 5 5 5 Antioxidant A 2 2 2 2 2Antioxidant B 2 2 2 2 2 Carbon black 40 40  40  40 40 Softener 5 5 5 5 5Bismaleimide Blend ratio 2 2 2 — 2 compound Type A A A — A Vulcanizingagent (Sulfur) — — — 3 3 Vulcanization accelerator (MBTS) 2 2 2 — —Vulcanization accelerator (CBS) — — — 1 — Vulcanization accelerator(TMTD) — — 1 1 — Vulcanization accelerator (MDB) — — — — — Workability ◯◯ ⊚ ⊚ ⊚ Initial Tensile strength at break 16 17  19  18 18 physical(MPa) property Elongation at break (%) 500 510  590  500 490 Hardness(JIS A) Change 56 56  56  57 57 Heat aging Change rate of elongation −19−20    −20    −70 −50 property at break (%) Hardness change (from the +1+1  +1  +7 +5 initial hardness) Spring change: 100% modulus +18 +18   +21    +80 +70 change rate (%) Durability Number of flexing cycles until170 200≦  200≦   100 130 the cracking size reaches 5 mm (×10000)

It was confirmed from the above-mentioned results that the compositionsof Examples 1 to 17 were excellent in workability, initial physicalproperty, heat aging property, and durability.

On the other hand, in the composition in Comparative Example 1, whichdid not contain a bismaleimide compound as the vulcanizing agent andcontained a sulfur-vulcanizing agent only, change rate of the elongationat break and hardness change after the heat aging and the spring changewere large; thus the vibration damping performance was poor. In thecomposition in Comparative Example 2, which contained a bismaleimidecompound as the vulcanizing agent and simultaneously contained thesulfur-vulcanizing agent, the spring change was large; thus thevibration damping performance was poor.

The vibration damping rubber compositions according to the presentinvention can be suitably used as vibration damping materials forengine-mounts, stabilizer bushings, and suspension bushings used invehicles such as automobiles.

1. A vibration damping rubber composition comprising: a diene-basedrubber; a vulcanizing agent of a bismaleimide compound selected from thegroup consisting of N,N′(4,4,-diphenyl-methane) bismaleimide representedby the following chemical formula (3), bis(3-ethyl-5-methyl-4-maleimidephenyl)methane represented by the followingchemical formula (4), 2,2′-bis [4-(4-maleimide phenoxy) phenyl]propanerepresented by the following chemical formula (5),m-phenylenebis(methylene) bismaleimide represented by the followingchemical formula (6), and m-phenylenebis (methylene) biscitraconimiderepresented by the following chemical formula (7),

 wherein the vulcanizing agent is sulfur-free; and a vulcanizationaccelerator.
 2. The vibration damping rubber composition according toclaim 1, wherein the vulcanization accelerator is a thiazole-basedvulcanization accelerator.
 3. The vibration damping rubber compositionaccording to claim 2, wherein the thiazole-based vulcanizationaccelerator is dibenzothiazyl disulfide or2-(4-morpholinodithio)benzothiazole.
 4. The vibration damping rubbercomposition according to claim 1, wherein the vulcanization acceleratoris thiuram-based vulcanization accelerator.
 5. The vibration dampingrubber composition according to claim 1, wherein a thiazole-basedvulcanization accelerator and a thiuram-based vulcanization acceleratorare used in combination.
 6. The vibration damping rubber compositionaccording to claim 1, wherein the diene-based rubber is at least oneselected from a group consisting of natural rubbers, isoprene rubbers,butadiene rubbers, styrene-butadiene rubbers, and acrylonitride-butadienrubbers.